This invention relates generally to radiotelephones, such as digital cellular telephones, also referred to herein as mobile stations, and relates more particularly to mobile station RF transmitter circuitry.
One major parameter of the transmitter of a mobile station (MS) is the Adjacent Channel Power Ratio (ACPR). It is related to the nonlinearity of the transmitter circuits, such as the power amplifier (PA), and it basically defines the maximum output power of the MS. In order to meet the maximum output power specification conventional practice designs the transmitter with a substantial margin in the ACPR at any output power level. The desired goal is for the transmitter to always meet the ACPR requirement at any power level despite component, temperature, battery voltage and other variations.
However, this is not an optimum technique with which to design a transmitter, as the additional safety margin results in an increase in DC current consumption. For battery powered devices any increase in DC current consumption is undesirable, as it reduces the amount of time between required rechargings of the battery. For a cellular telephone, this can translate into a reduction in both talk time and standby time.
Ideally, the transmitter""s bias current should be variable such that only a minimum DC current is consumed to produce a transmitted signal that just meets the ACPR specification at any given output power. Such a transmitter would require a mechanism that is simple enough to incorporate within the MS, and that would also be capable of measuring the ACPR quickly and continuously. Prior to this invention, such a mechanism was not known.
In greater detail, and referring to FIG. 3, the ACPR is defined as the dB difference between a signal""s main power (P0) and its xe2x80x9cadjacent channelxe2x80x9d power (P1) at a certain frequency offset from the signal""s center frequency. Let ACPRsig be the ACPR of the signal,
ACPR sig=P1xe2x88x92P0.xe2x80x83xe2x80x83(1)
ACPRsig will decrease as P0 increases. This is how ACPR determines the output power P0. For IS-95 and similar Code Division Multiple Access (CDMA) systems, the main desired channel power is located within a 1.23 MHz band around the channel center frequency. The adjacent channel has a bandwidth of 30 KHz and is offset by 900 KHz from the center frequency of the desired channel. The typical measurement of P0 and P1 is by the use of spectrum analyzer in the laboratory. Generally, such a measurement method is difficult, if not impossible, to implement in the MS.
Referring to FIG. 4, a diode power detector 20B is a common feature in the MS transmitter, and is used for power control purposes when operating in either the digital mode or the analog mode (e.g., the AMPS mode). The input to the diode power detector circuit 20B, obtained from a directional coupler 21, is normally situated at the output of the RF PA 20xe2x80x2 (the output going typically to a duplexer 20A). In a manner similar to that of an envelope detector in a conventional AM receiver, the power detector circuit 20B outputs a DC voltage that tracks the envelope of the RF signal. V_DC applied through R2 represents a forward bias potential for the power detector diode, and C1 and R1 are a part of the envelope detector. The RF signal is coupled to the anode of the power detector diode through a coupling capacitor (C2).
Before the power detector circuit 20B can be used to monitor the RMS power of the MS transmitter, it needs to be calibrated to establish the relation between its output voltage and the average power of CDMA signal to be detected. This is typically done in the factory when the MS is tuned.
When the detector is used to monitor the output power of the PA 20, only the DC component of its output is used. However, the time-varying envelope of the CDMA signal also contributes AC components to the output of the power detector circuit 20B. In conventional practice these AC components are considered undesirable for power monitoring purposes, and heavy lowpass filtering is generally used to remove these undesirable AC components.
It is a first object and advantage of the use of this invention to provide a technique to calculate the ACPR in real-time in the mobile station, and to use the results of the calculation to reduce the DC power consumption of the mobile station.
It is a further object and advantage of this invention to make use of the undesirable AC components in the output of the transmitter power detector circuit for making ACPR measurements.
The foregoing and other problems are overcome and the foregoing objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
The teachings of this invention provide measurement embodiments that fulfill the need to measure the ACPR with circuitry that can be incorporated into a mobile station. One important aspect of these teachings is that they enable the use of power detector circuitry that is already found in the mobile station transmitter circuitry, enabling the measurement of the ACPR by analyzing the output signal from the power detector, using an already present Digital Signal Processor (DSP), without incurring any incremental cost or circuit complexity.
A method is disclosed for operating a mobile station. The method includes steps of (A) operating an RF transmitter power detector to obtain an output signal that is indicative of a magnitude of RF power transmitted by the mobile station; (B) processing the output signal to determine a time-varying characteristic of the output signal; and (C) determining a value of the Adjacent Channel Power Ratio (ACPR) from the time-varying characteristic. A further step uses the determined value of the ACPR to control a DC bias current, and therefore the output power capability, of a transmitter amplifier.
In one embodiment the time varying characteristic of the output signal includes a frequency spectrum. In this embodiment the step of processing makes a comparison of the power in one portion of the frequency spectrum corresponding to a desired channel frequency to the power in another portion of the spectrum corresponding to an adjacent channel frequency. The step of processing includes a step of obtaining a Fourier transform of the output signal, or otherwise filtering the output signal, and integrating the result.
In another embodiment the time varying characteristic includes output power excursions from an average power level, and the step of processing obtains a ratio of the magnitude of peak power level excursions to the average power level during an interval of time.
In a related embodiment the output power excursions include positive and negative output power excursions from the average power level, and the step of processing obtains a ratio of the magnitude of the positive power level excursions to a magnitude of the negative power excursions during an interval of time.